1. Field of the Invention
The present disclosure relates to a semiconductor device, and more particularly, to a semiconductor device using a nitride to be used in circuits such as an inverter and a power supply circuit.
2. Description of the Related Art
A group III-V nitride compound semiconductor represented by gallium nitride (GaN), that is, a so-called nitride semiconductor has attracted attention. The nitride semiconductor is a compound semiconductor made of aluminum (Al), gallium (Ga) and indium (In) belonging to the group III, and nitrogen (N) belonging to the group V, and expressed by a general formula of InxGayAl1-x-yN (0≦x≦1, 0≦y≦1, x+y≦1).
With the nitride semiconductor, various mixed crystals can be formed and a hetero junction interface can be easily formed. In the hetero junction of the nitride semiconductor, a two-dimensional electron gas layer (2DEG layer) having a high concentration is generated in a junction interface through spontaneous polarization or piezoelectric polarization even in an undoped state. Thus, attention has been increasingly focused on a field effect transistor (FET) and a Schottky barrier diode (SBD) used as a high frequency or high power device in which this high-concentration 2DEG layer is used as a carrier layer.
However, a phenomenon called current collapse is likely to occur in the FET or SBD using the nitride semiconductor. The current collapse is the phenomenon in which when the device is turned on after it has been turned off once, a drain current is not likely to flow for a given length of time. Due to the current collapse, it is difficult to perform a switching operation at high speed, which causes extremely serious problems in a device operation.
As a method to prevent this current collapse from occurring, it is proposed to relax an electric field generated inside the device when a high voltage is applied to the device. For example, there is a method to relax an electric field at a gate end by forming a gate field plate in the FET (refer to Patent Literature 1).
Furthermore, it is said that along with relaxing the electric field, it is favorable to form a SiN protective film on an uppermost layer of a nitride semiconductor layer. This is because, when the SiN film is formed, it is possible to reduce a defect of an interface between the protective film and the nitride semiconductor layer, and prevent electrons from being trapped by the defect due to an intense electric field.